Because the boiling points of isomeric compounds are very close to each other, customary methods, such as distillation, often prove to be unsuitable for separation thereof. Distillation columns having a large number of trays are necessary because of these small boiling point differences. For sharp separations, the distillation should be carried out at a high reflux ratio, which leads to long residence times in the columns and, therefore, to a considerable exposure to heat of the material to be distilled. Exposure to heat usually results in a reduction in yield, and is particularly adverse in the case of sensitive compounds. Such compounds are discolored in a manner such that they cannot be decolorized by distillation; moreover, undesirable by-products are formed. Needless to say, distillation can be used as a separation method only on distillable substances. In contrast, substances which suffer degradation or thermal decomposition as a result of distillation are unsuitable for separation by this method.
Fractional crystallization, likewise a customary separation method, can also be used to only a limited extent. On the one hand, this process is limited to crystallizable substances and, on the other hand, fractional crystallization is highly labor-intensive. Moreover, it entails substantial expenditures on apparatus, in particular because of the numerous individual crystallization steps usually required.
The isolation and purification of methyl-branched saturated fatty acids having 14 to 24 carbon atoms by means of a particular variant of fractional crystallization is described in DE 38 07 401 A1. An aqueous solution of a wetting agent is added to the molten fatty acid mixture, and the mixture is allowed to crystallize while being stirred. This process is called hydrophilization, or fractional crystallization in the presence of wetting agents. The dispersion obtained by this process is centrifuged, separated into a lighter phase, containing the methyl-branched fatty acid largely free of wetting agent, and a heavier phase, consisting of wetting agent solution and crystals of straight-chain saturated fatty acids dispersed therein.
Specific methods for separation and concentration of aliphatic compounds are also used to a limited extent. Zimmerschied, Dinerstein, Weitkamp and Marschner thus describe, in Ind. Eng. Chem. 1950, 42 (7), pages 1300 to 1306, crystalline adducts of urea with linear aliphatic compounds. The formation of urea inclusion compounds depends on various factors, including the chain length and linearity of the aliphatic compound. However, the nature and size of the substituents, and the number and position thereof, also have an influence on the formation of the urea inclusion compounds. If the chain length is adequate, slightly branched alkanes also lead to urea adducts. Thus, for example, 2-methyloctadecane forms a urea inclusion compound, the stability of which is comparable to that of n-hexadecane. Similar behavior is also exhibited by esters of fatty acids; esters of methyl-branched carboxylic acids also form urea inclusion compounds (cf. page 1302, left-hand column to page 1303, left-hand column, second paragraph).
British Patent 1,240,513 describes the production of a therapeutically usable mixture consisting of esters of linoleic acid and .gamma.-linolenic acid. By addition of urea to a solution of esters of palmitic, stearic, oleic, linoleic, and .gamma.-linolenic acid in methanol, a mixture which is predominantly esters of palmitic, stearic, and oleic acid is precipitated as urea inclusion compounds thereof. The desired therapeutically usable mixture of esters of linoleic and .gamma.-linolenic acid is obtained by subsequently extracting the solution which remains with a suitable organic solvent. However, this procedure is limited to the separation of esters of non-branched saturated and unsaturated carboxylic acids.